1. Field of the Invention
The present invention relates to an electric parking brake device that performs an actuation and release of a parking brake by an electric motor, and to a release method therefor.
2. Description of Related Art
In Japanese Patent Application Laid-Open Publication No. 2012-229798, a disc brake including a parking brake mechanism is disclosed. This parking brake mechanism has a ball-and-ramp mechanism and a screw mechanism. Then, by rotation of an electric motor, the parking brake mechanism moves a piston by the ball-and-ramp mechanism and the screw mechanism, and holds the piston at the brake position by the screw mechanism.
Moreover, in Japanese Patent Application Laid-Open Publication No. 2012-101749, an electric parking brake that performs an actuation and release thereof by an electric motor and a cable is disclosed. In this patent literature, in an actuation state of the brake, the number of rotations of the electric motor is decreased before ending the actuation of the brake. The electric motor is driven by pulse width modulation (PWM) control, in which the number of rotations is controlled by a duty ratio of a drive pulse. In comparison with ON/OFF control, the PWM control can decrease the number of rotations of the motor, and accordingly, becomes an effective means for reducing operating noise of a reduction mechanism.
Incidentally, in the parking brake mechanism, a stroke amount is changed depending on a depression amount of the brake and a thickness of pad. Therefore, in an event of releasing the parking brake, it is necessary to detect a torque drop of a rotation-linear motion conversion mechanism that converts the rotation of the electric motor into a motion in a linear direction.
However, in the PWM control, current control is performed at a short time interval so that the electric motor can reach target torque, and accordingly, such a change that a drive current is greatly decreased stepwise is not generated. Therefore, the torque drop of the rotation-linear motion conversion mechanism cannot be detected. If a vehicle is moved while leaving the parking brake unreleasable, then it is possible that rear wheel dragging may occur.